[macOS] prevent the creating of /nix? about nix-installer

[angerman]

Just wanted to leave this here. It appears (after reading the code, and thinking it might work), this actually does work:

# install
curl --proto '=https' --tlsv1.2 -sSf -L https://install.determinate.systems/nix \
  -o /var/root/nix-installer
chmod +x /var/root/nix-installer
/var/root/nix-installer install macos \
  --volume-label "Nix Store" \
  --root-disk $(diskutil list |grep "Nix Store"|egrep -o '(disk\d+)') \
  --no-confirm

Using the --volume-label and --root-disk options combined on a on an unmounted apfs disk seems to have the desire effect.

[Hoverbear]

Unfortunately the dance of creating a Nix volume is annoyingly complicated as there are many states which the user system may be in which look fine but are otherwise broken. There has been some effort made to support this situation, however it's quite difficult and labor-intensive for me to test accurately as I don't use a Mac and our test Macs don't have external disks.

If you're interested, there is a create_nix_volume.rs inside src/actions/macos which composes together several other actions that make up the volume logic. Each of those actions has a plan function where we try to detect if the step is already completed.

If you run nix-installer plan > plan.json then review the plan.json the first entry in actions should be the volume creation stuff. You can see that some of those plan steps will be state: "Completed", those it's properly detecting are already done, however something to do with the volume creation is state: "Uncompleted". If you could figure out which one, we could possibly fix the issue.

As a hack/workaround, you can likely change the state of that first actions entry to Completed then run nix-installer install --plan plan.json and entirely skip volume creation.

There may also be issues around the launchd services we have to provision, as those are relatively tightly bound to the volumes we create, so we may not be out of the woods after this fix.